This invention relates, in general, to combined cycle power plants; and, in particular, this invention is directed to a means for raising gas turbine exhaust gas temperatures in a heat recovery steam generator upstream from a steam generating boiler tube section.
One application of the present invention is found in a heat recovery steam generator (HRSG) of a combined cycle power plant wherein gas turbine hot exhaust gases are passed through a duct containing a boiler tube section whereby a heat transfer is effected between the hot exhaust gases and the fluid in the boiler tube section to produce steam for driving a steam turbine. Moreover, it is sometimes desirable to raise the temperature of the hot exhaust gas, either intermittently or continuously, upstream from the boiler tube section in which case the HRSG is referred to as being "fired". One prior art means for firing an HRSG is a grid burner of the type shown in U.S. Pat. No. 3,830,620 to Frederick J. Martin, issued Aug. 20, 1974. In general, a grid burner includes a network of fuel supplied pipes which are disposed within and across the cross-sectional area of the HRSG duct. Each pipe includes a plurality of fuel outlet ports on the downstream circumference of the pipe. In a grid burner, fuel is delivered through the grid pipes and ignited at the outlet ports on the downstream circumference of the pipe by pilot burners which are mounted transversely and adjacent to the grid pipes downstream therefrom.
One limitation inherent in the grid burner system is the possibility that the fuel pipes in the grid burners may become plugged. This is because temperature affects fuel viscosity and the fuel pipes are disposed in the HRSG duct which is subject to wide temperature variations dependent upon gas turbine operating conditions. Solutions to this problem are available in the form of purging means and/or insulation means for the fuel pipes; and, limiting the fuel used to distillate oils or natural gas. In the event that fuel pipe clogging should occur, maintenance requires access to the HRSG duct or, in the alternative, a solution such as that shown in U.S. Pat. No. 3,843,309 to Lambiris, issued Oct. 22, 1974 wherein the fuel pipe is removable from the burner pipe.
According to the present invention, the disadvantages of the grid burner system are obviated by providing a wall burner system for heating gas turbine exhaust gases whereby all fuel delivery means are disposed outside the HRSG duct. The wall burners are provided in combination with flameholders which are disposed within the HRSG duct but the flameholders do not contain any fuel delivery pipes and therefore are not susceptible to fuel pipe clogging. Moreover, the wall burners may be operable on relatively inexpensive residual fuel oils. The wall burners may be directed upstream into the exhaust gas flow and each wall burner includes an associated vee-shaped flameholder which may also be directed upstream into the exhaust gas flow approximately parallel to the burner axis. Ignition occurs within the wall burner and the flameholder attracts the emitted flame along the flameholder length thereby preventing the flames from being lifted downstream to the boiler tube section by the turbine exhaust gas flow. The directing of the wall burner upstream into the exhaust gas flow allows the flame length to be maximized across the HRSG duct. Moreover, a shorter duct is required between the combustion section and the boiler tube section since the flames are directed away from the boiler tubes.
It is one object of the present invention to provide a means for firing an HRSG which obviates the occurrence of fuel pipe clogging.
It is another object of the present invention to provide a means for firing an HRSG which may be operated on residual fuels.
It is another object of the present invention to provide a means for firing an HRSG which is more economical to construct, operate and maintain than prior art devices.
The novel features believed characteristic of the present invention are set forth in the appended claims. The invention itself, however, together with further objects and advantages thereof, may best be understood with reference to the following description taken in connection with the appended drawings.